The injection of displacement fluids into subterranean hydrocarbon-bearing formations to promote the production of hydrocarbons therefrom is well known. Water and various gases, in addition to more complex fluids, such as surfactant solutions and polymer solutions, are common displacement fluids utilized in both miscible and immiscible oil displacement floods.
When a displacement fluid is injected into a formation has high and low permeability flow paths, the fluid be substantially diverted away from the low permeability flow paths. As a consequence, the displacement fluid does not sweep the low permeability flow paths and the sweep efficiency of the displacement fluid in the formation is poor. Conformance control techniques are employed to improve the sweep efficiency on a macroscopic scale, i.e., by effectively blocking the high permeability flow paths, such as fissure/fracture networks.
Polymer gels have been widely used for conformance control of naturally fissured/fractured reservoirs. For an overview of existing polymer compositions, reference is made to the U.S. Pat. Nos. 5,486,312 and 5,203,834 which also list a number of patents and other sources related to gel-forming polymers.
In an effort to reduce the cost of the gelling system without substantially diminishing the effectiveness of the treatment, attempts are known to at least partially substitute the polymer by a less expensive component. One way, which is of specific interest with regard to the present invention, are foamable gel compositions as described for example in the U.S. Pat. Nos. 5,105,884, 5,203,834, and 5,513,705, wherein the polymer content is reduced at constant volume of the composition.
The typical components of a foamable gel composition are (a) a solvent, (b) a crosslinkable polymer, (c) a crosslinking agent capable of crosslinking the polymer, (d) a surfactant to reduce the surface tension between the solvent and the gas, and (e) the foaming gas, itself.
U.S. Pat. No. 5,105,884 describes foamed gel systems based on partially hydrolyzed polyacrylamide PHPA crosslinked with Cr(III) acetate. With regard to the foaming gas, all four examplatory systems employ nitrogen (N.sub.2), though CO.sub.2 is mentioned as an alternative.
The use of CO.sub.2 as foaming gas is desirable from an economic viewpoint, as this gas is used in many gas injection projects designed to generate an external fluid drive in the reservoir. Therefore an economic source of CO.sub.2 would in principle be available for the gel foaming step. However, experiments with known gel systems showed that CO.sub.2 when used as foaming gas has a considerable impact on the stability of the gelling system. In contrast to N.sub.2, CO.sub.2 exhibits significant solubility in both water and oil. When CO.sub.2 dissolves in water, it is converted to carbonic acid. It was found that known formulations for gelling systems either failed to gel in the presence of CO.sub.2 gas or resulted in a gel with reduced long-term stability.
It is therefore an object of the invention to provide a CO.sub.2 foamable gelling composition. It is a particular object of the invention to provide a cost-efficient composition with sufficient long-term stability to be useable in conformance control applications.